Carbon fiber reinforced polymer composites are known for their outstanding mechanical properties with a low density. Some of their outstanding mechanical properties include superior shear properties and impact resistance. For this reason, they have been of interest to many fields, particularly for rugged applications, such as the space and aeronautics industries, military equipment, transportation, and infrastructure.
However, a key deficiency in these composite materials has been an insufficient adhesion of the carbon fibers to the polymer matrix. In turn, the low adhesion has a detrimental effect on the mechanical properties, usable lifetime, and integrity of such composites. Although several methodologies, including electrolytic (i.e., electrochemical) treatments or the use of various sizing agents on the carbon fiber, have been investigated in attempting to improve the surface characteristics of carbon fibers, they have been largely inefficient for achieving the desired end, or impractical from a cost or processing standpoint.
Electrochemical treatment of carbon fiber is commonly used in the art, as described, for example, in U.S. Pat. Nos. 3,671,411, 3,832,297, 3,859,187, and 4,050,997. In a typical electrochemical treatment process, carbon fiber is passed through a conductive solution while current is applied to the fiber. However, electrochemical treatment has several drawbacks. In particular, the electrolytic solution used in the electrochemical treatment generally requires a significant degree of maintenance, i.e., checking and controlling the levels of electrolyte and other species, filtering away debris, and recirculation. Eventually, the electrolytic medium will become spent, at which point it is typically discarded and replaced with new electrolytic liquid, all of which results in increased cost and possible detriment to the environment.
Another significant disadvantage of the electrochemical process is its general reliance on a network of rollers that guide the fiber tow or fabric in a serpentine path through the electrolyte. Moreover, once the bundle of filaments exits the electrolyte bath, the filaments are typically passed through a nip roller that squeezes out residual liquid from the fiber. Particularly for high modulus carbon fiber that possesses a degree of brittleness or low elasticity (as is typical for carbon fibers that have been graphitized), the bending and squeezing mechanical action of the rollers subjects the carbon fiber to an excessive level of stress and strain that ultimately causes damage to the carbon fiber.